Tables :: Fill One Field Based On Another

I'm creating an accounting database for my use at work. I order goods and services from a multitude of vendors, using varying payment methods, and at last count, 8 different accounts. The time I'm spending on repetitive entry into excel spreadsheets has forced me to sit down and make an access database to track everything.

Right now, the following:
1) Table named "Accounts" with two fields - Account # and Account Description. Account # is PK.
2) Table labeled "Orders", which I'm going to have the top-level information for each order (order #, Account #, Account description, + others that don't matter here). I have another table for the order details (product #, description, price...).
3) Relationship between "Accounts" and "Orders" is one-to-many, enforced referential integrity, cascaded update.

For my order entry form, I want to enter the account description, so I set up a combobox. But, for my paperwork, I am going to need the account # to print when I do a report, and for future flexibility I want that field to remain in the "Orders" table.

I know my accounts by name, not number, and I want it set up so that when I select, for instance, "Supplies Account" for the description on the form, the Account # field on the Orders table is automatically populated with the account # that correlates to that description.

Is there a way to tell the table to auto-fill this entry?